In recent years, in the field of optical communication or optical measurement, a waveguide type optical element such as an optical modulator having optical waveguides disposed on a substrate having an electro-optic effect has been frequently used. Generally, a waveguide type optical element has the optical waveguide and an electrode for controlling optical waves that are transmitted through the optical waveguide.
As the above-described waveguide type optical element, for example, a Mach-Zehnder-type optical modulator in which lithium niobate (LiNbO3) (also referred to as “LN”), which is a ferroelectric crystal, is used as the substrate is widely used. A Mach-Zehnder-type optical modulators has a Mach-Zehnder-type optical waveguide consisting of an input waveguide for introducing light from outside, a branching section for transmitting the light introduced through the input waveguide to divide into two paths, two parallel waveguides for transmitting the respective branched lights toward latter part of the branching section, and an output waveguide for combining the lights that are transmitted through the two parallel waveguides and outputting to outside. In addition, a Mach-Zehnder-type optical modulator has an electrode for shifting and controlling the phases of optical waves that are transmitted through the parallel waveguides using an electro-optic effect by applying voltage thereto. Generally, the electrode is constituted of a radio frequency (RF)-signal electrode disposed on or above the parallel waveguides or near the parallel waveguides (hereinafter, referred to as “RF electrode”) and ground electrodes disposed away from the RF electrode.
In a Mach-Zehnder-type optical modulator in which LN is used as the substrate, the optical output characteristics with respect to applied voltage shifts due to a so-called DC drift phenomenon or temperature drift phenomenon, and thus, for example, optical modulation waveforms output from the modulator are distorted, and change of the modulation characteristics may occur (for example, deterioration of the waveform quality).
As a method for preventing the change of the modulation characteristics caused by those drift phenomena, a method in which, in addition to the RF electrode and the ground electrode for applying a radio-frequency signal voltage, a bias electrode is disposed along the parallel waveguides, and a suitable voltage is appropriately applied to the bias electrode, thereby compensating for the amount of voltage shift caused by the above-described drift phenomena (hereinafter, also referred to as “DC drift voltage”) is known (Patent Literature 1).
That is, the amount of voltage shift is adjusted by generating an appropriate refractive index difference between the two parallel waveguides by applying a voltage to the bias electrode.
In addition, as a technique for diminishing the drift phenomenon, it is known that a buffer layer is made of a transparent insulator of a mixture between silicon oxide and an oxide of or an oxide of at least one further element selected from the group consisting of the metal elements of the Groups III to VIII, Ib and IIb of the Periodic Table or an oxide of at least one further element selected from the above-mentioned metal elements and semiconductor elements (Patent Literature 2). This constitution is a useful technique since negative DC drift characteristics appear in the beginning with lapse of time and, additionally, movement of movable electrons and ions is affected by the additives, and thus it is possible to flatten an increase of DC drift, compared with conventional one, so that DC drift characteristics are improved for a long period of time.
However, in the above-described related art, in a case in which the RF electrode and the bias electrode are individually disposed along the parallel waveguides with a length limited by size and the like of the waveguide type optical element, the length of the RF electrode makes longer in order to lower the half-wavelength voltage (Vπ) in a Mach-Zehnder-type optical modulator and, as the result, there is a case in which the length of the bias electrode becomes shorter. In such a case, the electric field required to generate a desired refractive index difference between the parallel waveguides becomes larger, and thus the applied voltage to the bias electrode also becomes higher.
As a result, a phenomenon in which the above-described drift phenomenon is accelerated due to a high electric field applied to an LN substrate through the bias electrode may occur.